Certain cars are equipped with so-called airbags as an alternative or as a supplement to standard vehicle safety harnesses. These bags are swiftly inflated in front of the driver or passengers of the vehicle in the event of a collision, so as to shield the driver or passengers from such injuries as those which might be inflicted when thrown forwards against the steering wheel or the instrument panel.
Hitherto, vehicles have not been equipped with airbags for backseat passengers. The reason is possibly because backseat passengers would require much larger bags, which is probably associated with a number of problems which can be related purely to gas kinetics.
In the event of a head-on collision, or any other collision, in which the vehicle is stopped very quickly, it is necessary for the container to be inflated with gas and form an airbag or a gas cushion before the person is thrown forward against the backrest of the front seat or against the steering wheel/or the instrument panel at the initial speed of the vehicle.
In order to obtain effective protection, it is estimated that the airbag must be inflated within one-hundredth of a second.
Assuming that a car is travelling at a speed of 110 km/hour when a collision occurs, a person seated in the car will be moved through a distance of about 0.3 meter in relation to the ground in this time. Since the actual body of the vehicle will not stop instantaneously, owing to the fact that the front of the vehicle is buckled-in through a distance of one or more decimeters, the driver or passengers of the vehicle have time to move about one to two decimeters relative to the vehicle body in 0.01 seconds, which means that the airbag has time to inflate and to prevent the driver or passengers being thrown onto the driving wheel, the instrument panel or the backrest of the seat in front.
A device for filling a flexible container (airbag) essentially instantaneously with gas upon the occurrence of powerful retardation forces is known from DE-A1-2061759. The device comprises a flexible container to be filled with gas, an elongated pressurized vessel containing a gas, e. g. nitrogen or helium, a channel connecting the flexible container with the pressurized vessel and an explosive charge arranged to open the channel between the vessel and the container when the container is to be inflated. This reference mentions a suitable gases nitrogen and helium, both being preferred among other gases in general.
When a gas, which in case of nitrogen expands without the exchange of energy (iso-enthalpy), the temperature will normally fall in accordance with the so-called Joule-Thomson effect.
In the case of air or nitrogen, which is permitted to expand from 400 to 2 bars, the temperature will fall by about 70 to 100 degrees. When, as in the case of airbags, the gas is stored in high pressure containers, it is either necessary to supply corresponding heat to the gas so as to obtain the volume that would be obtained at room temperature, or the pressure container must be filled with about 30% more gas.
In addition to this drop in temperature, it must be remembered that air, nitrogen and other gases have a compressibility factor of about 1.2 at 400 bars, which must be taken into account when dimensioning the container, since otherwise the pressure or the volume must be increased to a corresponding degree in accordance with the case of an ideal gas.
Another problem associated with all heavy gases is that, in the present case, the outflow velocity of the gas would be relatively low, since this gas velocity is a function of both molecular weight and temperature.